This invention relates to a method of deriving radio-frequency impedance information of a network by applying radio-frequency energy to the network, extracting signals at radio frequency from the network and deriving the impedance information with the aid of a detector to which the extracted signals are fed. The invention also relates to apparatus for carrying out such a method. The method and apparatus can be used, for example, to match a radio transmitter or receiver to an aerial.
Such an arrangement is disclosed, for example, in British Pat. No. 1,330,016 which relates to impedance-transforming apparatus for matching a load to a power source and in particular for matching an aerial to a radio transmitter. The specification describes an arrangement which includes a detector which, when energy is supplied by the transmitter to the aerial, senses the magnitude and phase angle of the aerial impedance as transformed by a tuning unit connected between the transmitter and the aerial. The data derived by the detector are fed to a servo amplifier which controls motors mechanically coupled to a variable inductor and a variable capacitor in the tuning unit.
Automatic matching of an aerial to the output of a transmitter is also known for example from British Pat. Nos. 881,018 and 1,412,314. A technique for adjusting components of an aerial matching unit is described by M. J. Underhill and P. A. Lewis in "Automatic tuning of antennae," SERT Journal, September 1974, page 183.
In previous arrangements such as those referred to above it is necessary to supply the load with a sufficiently large amount of energy to permit the detector or detectors to function adequately.